1. Field of the Invention
The present invention relates to a rear wheel steering control system for automotive vehicles, wherein rear wheels of a vehicle are steered during a main steering operation for front wheels performed by driver's manual operation of a steering wheel.
2. Description of the Prior Art
Conventionally, various types of rear wheel steering control systems are known, including a so-called steering angle responsive type as disclosed, for example, in JP-B2-60-44186. This particular type of control system is featured by such a control that, the rear wheels are steered (i) in the same-phase direction as the front wheels for a relatively small manual operation angle of the steering wheel, and (ii) in the opposite-phase direction as the rear wheels for a relatively large steering wheel operation angle.
More particularly, FIG. 5 represents typical rear wheel steering angle characteristics which can be achieved by the steering angle responsive type control system. Here, the rear steering angle .delta..sub.r assumes the maximum same-phase angle when the steering wheel manual operation angle .theta. is .theta..sub.0. Upon further increase in the steering wheel operation angle, different rear wheel steering angle characteristics are selectively achieved depending upon whether the same-phase rear wheel steering angle begins to decrease toward the opposite-phase angle either at an increased steering wheel operation angle .theta..sub.1 (the solid line a in FIG. 5), or at a further increased steering wheel operation angle .theta..sub.2 (the imaginary line b in FIG. 5). For the sake of convenience, the rear wheels steering characteristics as represented by the solid line and the imaginary line in FIG. 5 may be referred to hereinafter, as "characteristic a" and "characteristic b" respectively.
Another type of rear wheel steering control system is disclosed, for example, in JP-A-57-11173 which is a so-called vehicle speed responsive type. Basically, this type of system is to perform the rear wheel steering so as to achieve a ratio K.sub.r (=.delta..sub.r /.theta.) as shown in FIG. 7, of the rear wheel steering angle .delta..sub.r in relation to the steering wheel operation angle .theta. which represents the front wheel steering angle .delta..sub.f. Thus, as a function of an actual vehicle speed V and with reference to the front wheel steering angle, the rear wheels are steered with an opposite-phase angle in a low speed range (V&lt;V.sub.0) and with a same-phase angle in a high speed range (V.gtoreq.V.sub.0).
As known in the art, front wheel drive type vehicles generally exhibit an understeering tendency which is particularly significant in a higher speed range wherein the vehicle body is subjected to a higher magnitude of lateral acceleration. A same-phase steering of the rear wheels in a high speed range V.gtoreq.V.sub.0 often results in an enhanced understeering tendency of the front wheel drive type vehicles, thereby significantly deteriorating the maneuverability characteristics of the vehicle. Thus, in the case of front wheel drive type vehicles, some drivers may operate the steering wheel by an increased operation angle .theta., corresponding to the enhanced understeering tendency for the same-phase steering region in the high speed range V.gtoreq.V.sub.0.
With this in mind, one approach to mitigate the enhanced understeering tendency may be to steer back the rear wheels toward the opposite-phase angle. This is achieved by setting a predetermined rear wheel steer-back angle .DELTA..delta..sub.r which assumes an incremental amount with increase in the steering wheel operation angle .theta. in a large steering wheel operation angle range, and also with increase in the vehicle speed V, both corresponding to increase in lateral acceleration of the vehicle. The steer-back angle .DELTA..delta..sub.r determines the rear wheels steering angle .delta..sub.r, as .delta..sub.r =K.sub.r .multidot..theta.-.DELTA..delta..sub.r.
The above-mentioned steering angle responsive type control system has either one of the characteristics a and b shown in FIG. 5, which has already been selected during the design stage, and the vehicle speed responsive type control system has one definite characteristic. Thus, these prior art systems may encounter the following problems.
There is shown in FIG. 6 different modes of the rear wheel steering angle variation upon a lane-change steering wheel operation with an operation angle .theta. which is variable with time as shown, respectively for the rear wheel steering angle characteristics a and b as explained with reference to FIG. 5. Here, the variations a.sub.1 and b.sub.1 correspond to the characteristics a and b, respectively. The hatched regions in FIG. 6 are regions wherein the steering wheel operation angle .theta. is in excess of respectively predetermined criteria .theta..sub.1, .theta..sub.2 so that the same-phase rear steering angle is reduced from the maximum angle .delta..sub.rm, or the rear wheel steering is changed from the same-phase steering toward the opposite-phase steering.
The presence of such hatched regions in FIG. 6 means that, upon manual steering back during a steering wheel operation for changing the lane, rear wheels are steered so that the same-phase steering angle is instantaneously once reduced from the maximum angle .delta..sub.rm and then increased to the maximum angle .delta..sub.rm again. In this sense, insofar as the lane change of the vehicle is concerned, the rear wheel steering characteristic b is generally considered superior to the characteristic a, because of a smaller hatched region b.sub.1 in FIG. 6.
From another viewpoint, however, the rear wheel steering characteristic b tends to give rise to a difficulty during a cornering of a vehicle, in particular during a high speed cornering, in effectively mitigating the above-mentioned enhanced understeering tendency of front wheel drive type vehicles. Such a difficulty is considered due to the smaller hatched region of the same-phase rear wheel steering characteristic variation as shown at b.sub.1, and hence due to a reduction of the same-phase rear wheel steering angle from the maximum angle .delta..sub.rm, at a retarded timing and with a poor reduction amount. As the case may be, it becomes almost impossible to mitigate the enhanced understeering tendency of front wheel drive type vehicles, due to the ground which will be explained below.
That is to say, in the case of the rear wheel steering characteristic b, the same-phase rear steering angle is maintained at the maximum angle .delta..sub.rm for a relatively long period so that a cornering characteristic of the vehicle, which is comparable with that for the rear wheel steering characteristic a, can be achieved only with the driver's manual operation of the steering wheel by an increased angle, and this is usually the case. However, front wheel drive type vehicles tend to exhibit enhanced understeering tendency during a high speed cornering, and undergoes understeering before the steering wheel is operated by an angle .theta..sub.2 where the same-phase rear steering angle begins to be reduced from the maximum angle .delta..sub.rm. On such occasion, the driver feels a reduced front wheel cornering power, or an increased cornering radius, and refrains from further increasing the steering wheel operation angle .theta.. As a result, the driver's manual steering wheel operation is actually performed within a range of .theta.&lt;.theta..sub.2, as shown in FIG. 6, and it is impossible to mitigate the enhanced understeering within a range of .theta..gtoreq..theta..sub.2. In this sense, insofar as the cornering of the vehicle is concerned, the rear wheel steering characteristic a is generally considered superior to the characteristic b, because of a larger rear wheel steering angle variation of the hatched region al in FIG. 6.
With the conventional rear wheel steering control systems as discussed above, due to a single kind of fixed rear wheel steering angle characteristic as determined in the design stage, both the characteristic demand for the lane change of the vehicle and that for the cornering of the vehicle could not be satisfied on compatible basis, and either one of the demand could be satisfied with the sacrifice of the other.
With the vehicle speed responsive type rear wheel steering control system, moreover, the rear wheel steering angle is basically determined to have a characteristic as shown in FIG. 7, and is modified in the direction of reduced same-phase steering angle (or increased opposite-phase steering angle) by an amount .DELTA..delta..sub.r as shown in FIG. 8. This gives rise to a problem that the rear wheel steering angle variation with the steer-back amount .DELTA..delta..sub.r degrades a desired stable maneuverability of the vehicle particularly during a lane change wherein the steering wheel is rapidly operated. On the other hand, an excessively small steer-back amount .DELTA..delta..sub.r would not permit an intended mitigation of an enhanced understeering of front wheel drive vehicles to be satisfactorily achieved during a lane change.